1. Field of the Invention
The present invention relates to a semiconductor device in which a thickness of an energy barrier of a Schottky junction is modulated by an electric field of an insulated gate electrode to control a tunnel phenomenon, thereby controlling a main current.
2. Description of the Background Art
In FIG. 1, there is shown a conventional semiconductor device such as a Schottky tunnel transistor, a thickness of an energy barrier of a Schottky junction is modulated by an electric field of an insulated gate electrode to control a tunnel phenomenon, thereby controlling a main current, as disclosed in Japanese Patent Laid-Open Specification No. 62-274775.
In FIG. 1a, an n.sup.+ -type drain region 2 is formed in the surface area of an n-type silicon semiconductor substrate 1, and apart from the drain region 2 a Schottky metal 3 for acting as a source region is also embedded in the surface area of the substrate 1 so as to form a Schottky junction between the substrate 1 and the Schottky metal 3. A gate electrode 5 is formed on the surface of the substrate 1 via a gate insulating film 4 formed thereon between the drain region 2 and the Schottky metal source region 3.
In FIGS. 1b to 1c, there are shown energy band structures of the Schottky junction and the bias states against the n.sup.+ -type drain region 2 and the gate electrode 5 in the surface area of the n-type silicon semiconductor substrate 1 of the Schottky tunnel transistor shown in FIG. 1a.
As shown in FIG. 1b, when both gate voltage V.sub.G and drain voltage V.sub.D are zero, the thickness W of the Schottky barrier is thick, and thus there is no electron flow between the drain and the source. In FIG. 1c, when V.sub.G =zero and V.sub.D &gt;zero, the thickness W of the Schottky barrier is thick, and a reverse bias voltage is applied to the Schottky junction, with the result of no electron flow between the drain and the source. In FIG. 1d, when V.sub.G &gt;zero, the energy band is largely bent by the electric field of the gate electrode 5, and the thickness W of the Schottky barrier becomes thin. Hence, when V.sub.D &gt;zero, electrons flow from the Schottky metal 3 to the semiconductor substrate 1 through the Schottky junction therebetween by the tunnel effect, and thus a tunnel current flows from the drain region 2 to the Schottky metal source region 3 through the Schottky junction. In FIG. 1e, when V.sub.G =zero and V.sub.D &lt;zero, a forward bias voltage is applied to the Schottky junction. As a result, a lot of electrons can move from the semiconductor substrate 1 to the Schottky metal 3, and thus an electric current flows therebetween in the forward-direction.
In this case, the tunnel current can be changed by adjusting the gate voltage V.sub.G. In a Schottky tunnel transistor utilizing this phenomenon, no punch-through occurs unlike a usual MOSFET, and thus the Schottky tunnel transistor is desired to be employed in a miniaturized device in the future.
However, in the abovementioned Schottky transistor, there is a problem in that a leak current is very large. This is explained as follows. In the conventional MOSFET, a barrier thickness of a pn junction between a source region and a substrate is thick, such as approximately 1000 .ANG.. When the gate voltage is zero and a reverse bias is applied to the drain only a diffusion current flows through the pn junction, and the leak current flowing through the junction is very small. On the other hand, in the Schottky tunnel transistor, when V.sub.G =0 and a reverse bias is applied to the drain as shown in FIG. 1c, the leak current IL of the Schottky junction is due to a thermionic emission beyond the triangular potential barrier height .phi.B and thus IL exponentially increases with the temperature according to the following formula, EQU IL.sup.b .congruent.cxp(-.phi.B/kT)
wherein k is boltzmann's constant and T is an absolute temperature.
However, in a conventional Schottky tunnel transistor, although the effective source region, of which the tunnel current is affected by the gate voltage V.sub.G, is only a part of the Schottky junction near the gate electrode, the Schottky junctions are widely formed in the area where the substrate 1 and the Schottky metal 3 contact each other. Accordingly, when the temperature and/or the drain voltage V.sub.D increase, the leak current of the Schottky junctions becomes many times larger than that of the effective source region, and furthermore, since the drain voltage is directly applied to the Schottky junction, the curve of the triangular potential at the Schottky junction becomes sharp, thereby equivalently bringing about the depression of the Schottky barrier .phi.B. As a result, in this respect, the leak current increases, and thus the blocking voltage is reduced.